This application claims the benefit of United Kingdom Patent Application Number 0103541.9, filed Feb. 13, 2001.
This invention relates generally to fuse element or fuse link assemblies, and, more particularly, to fuse element assemblies for General Purpose or Full-Range fuses.
Fuses are widely used as overcurrent protection devices to prevent costly damage to electrical circuits. Fuse terminals typically form an electrical connection between an electrical power source and an electrical component or a combination of components arranged in an electrical circuit. One or more fusible links or elements, or a fuse element assembly, is connected between the fuse terminals, so that when electrical current through the fuse exceeds a predetermined limit, the fusible elements melt and opens one or more circuit through the fuses to prevent electrical component damage.
General Purpose or Full-Range type high voltage, current-limiting fuses are operable to safely interrupt both relatively high fault currents and relatively low fault currents with equal effectiveness. At least one type of General Purpose or Full-Range type fuses employs a fuse element assembly having two distinct portions. One portion is configured for opening of an electrical circuit under relatively low fault current conditions and a second portion is configured for opening of an electrical circuit under relatively high fault current conditions. The first portion includes a plurality of fuse elements contained in respective insulating sleeves and including a weak spot and/or low melting alloy spot located approximately at the center or midpoint of each of the fuse elements. The second portion includes a plurality of fuse elements fabricated from a high conductivity metal and connected in parallel with one another. The first and second fuse element portions are serially wound onto an insulating former and embedded in a arc-extinguishing material within a fuse body.
Under high fault current conditions, the second portion of the fuse element assembly partially vaporizes, and the arc extinguishing material absorbs energy and attains a high electrical resistance to safely and effectively interrupt current through the fuse. Under low fault current conditions, the first portion of the fuse element assembly interrupts current by melting of a fuse element within one or more of the insulated sleeves. The resultant arc within the sleeves generates ionized gas which is expelled from the open ends of the sleeves.
In elevated voltage and current applications, however, such as for protection of increasingly common 12 kV transformers with ratings as high as 1000 kVA, conventional Full-Range fuses have been found deficient. As current ratings and voltage ratings of Full-Range fuses are increased, the fuse is prone to undesirable internal and external damage from resultant increased energy of ionized gas blasts in operation of the fuse. While reinforcement of the insulating sleeves of the first portion of the fuse element assembly is of some use in producing higher current ratings and voltage ratings of Full-Range fuses, reinforcement of the sleeves tends to complicate assembly and increase manufacturing costs of the fuses without overcoming problematic excessive ionized gas blasts and resultant damage during operation of the fuse.
In addition, while voltage and current ratings of Full-Range fuses may be increased by using fuse elements and fuse constructions of greater cross sectional area and capacity, this increases the physical size of the Full-Range fuse. Especially when a large number of fuses are employed, increasing the size of the fuses is problematic.
In an exemplary embodiment of the invention, a fuse element assembly for a Full-Range fuse includes an insulative former having opposite first and second ends. A first electrically conducting connector is coupled to the first end of the former and a second electrically conducting connector is coupled to the second end of the former. At least one fuse element extends between the first connector and the second connector about the insulative former. The fuse element includes a low current interrupting fuse element portion extending from the first connector, a high current limiting fuse element portion extending from the second connector, and the low current interrupting fuse element portion and the high current limiting fuse element portion coupled to one another intermediate the first and second connector. An insulative sleeve surrounds the low current interrupting fuse element portion, and each sleeve includes a first end adjacent the first connector and a second end adjacent the high current limiting fuse element portions. The low current interrupting fuse element portion includes a weak spot located adjacent to but within the second end of a respective one of the sleeves. Alternatively, the weak spot is located in a range from 0 to 25% of the length of the sleeve as measured from the second end of the sleeve.
By locating the weak spot of the low current interrupting fuse element at an end of the insulating sleeve opposite the connector from which the low current interrupting fuse elements extend, ionized gas blasts generated in operation of a fuse is directed predominately toward a center of the fuse rather then the ends of the fuse near the end-caps. Therefore, by more efficiently and effectively expelling ionized gas from the insulative sleeve, the fuse element assembly avoids damage to the fuse body and end-caps that has been observed in conventional fuses, and higher voltage and current ratings are facilitated without increasing dimensions of fuse components. Thus, a superior performing Full-Range fuse is provided in a compact, space-saving construction in comparison to known Full-Range fuses.